The present invention relates to a demultiplexer for separating signal light, which has been densely wavelength-multiplexed within a relatively narrow wavelength range, into multiple optical signals corresponding to their respective wavelengths and outputting those demultiplexed signals. The present invention also relates to a demultiplexer-receiver for receiving and demultiplexing wavelength-multiplexed signal light and then converting the resultant optical signals into electrical signals.
In the field of fiber-optics communications, a technique of increasing the information-carrying capacity by utilizing wavelength division multiplexing (WDM), by which a plurality of optical signals corresponding to mutually different wavelengths are combined into a single signal, is well known. Especially in recent years, a system for multiplexing four waves with respective wavelengths around 1.55 xcexcm (each pair of which are different from each other by 3.2 nm) or even eight waves (each pair of which are different from each other by 1.6 nm) is on the verge of being implemented. And yet research and development is vigorously carried on to realize a super-high-density fiber-optics WDM network in the near future by reducing the wavelength difference to as small as 0.8 nm. Generally speaking, though, in a WDM telecommunications network, an optical signal, which has once been multiplexed on the transmitting end, should be demultiplexed on the receiving end. Accordingly, to realize a super-high-density WDM network like this, demultiplexing must be performed at a very high resolution. It is not impossible to realize that high-resolution demultiplexing using a spectroscope including a diffraction grating. However, a more cost-effective alternative would be constructing a system including either small-sized demultiplexers or an optical receiver module with those demultiplexers integrated on a semiconductor substrate, for example.
A known optical receiver module of this type, i.e., a module with demultiplexers integrated on a semiconductor substrate, is disclosed in Japanese Laid-Open Publication No. 8-46593, for example. Hereinafter, the optical receiver module will be described with reference to FIG. 8.
FIG. 8 illustrates a cross-sectional structure for a demultiplexing and light-receiving portion of an optical receiver module as disclosed in the Japanese Laid-Open Publication No. 8-46593 identified above. As shown in FIG. 8, a vertical cavity filter 105 is formed as a stack of lower and upper reflectors 102 and 104 and spacer layer 103 on the principal surface of a semiconductor substrate 101. Each of the lower and upper reflectors 102 and 104 and spacer layer 103 is formed out of a semiconductor layer. On the filter 105, multiple receivers 106 are formed just like the same number of islands. And a reflective film 107 is formed on the backside of the substrate 101 opposite to the principal surface thereof.
The filter 105 shows a transmittance of 100% against an incoming light beam with the same wavelength as one of the resonant wavelengths of the filter 105. In this structure, each of the resonant wavelengths is determined by the uneven thickness of the spacer layer 103. In addition, high-reflectance wavelength bands, termed xe2x80x9cstop bandsxe2x80x9d, exist around each resonant wavelength. An incoming wavelength-multiplexed light beam 109, which has traveled through an optical fiber bundle 108, impinges onto the backside of the substrate 101. And only a part of the light beam 109, of which the wavelength is equal to one of the resonant wavelengths, can be transmitted through the filter 105 and incident onto associated one of the receivers 106. The remaining part of the light beam 109, which has been reflected off from the filter 105, is reflected by the reflective film 107 and then incident onto the filter 105 again. The thickness of the spacer layer 103 is not constant but changes horizontally, i.e., relative to the principal surface of the substrate 101. Thus, the filter 105 has multiple resonant wavelengths for the respective receivers 106. As a result, optical signals corresponding to mutually different wavelengths are received one after another.
A resonant wavelength of the vertical cavity filter 105 is given by
2nLxc2x7cos xcex8/m
where n is a refractive index of the spacer layer 103, L is the thickness of the spacer layer 103 at a given point of incidence, xcex8 is an angle of incidence of the incoming light beam 109 (i.e., an angle formed by the light beam 109 with a normal of incidence perpendicular to the principal surface of the substrate 101) and m is a natural number. Accordingly, if the angle xcex8 of incidence of around 20 degrees changes by 1 degree, then the resonant wavelength of the filter 105 will change by about 0.63%. For example, when the resonant wavelength is around 1.55 xcexcm, the change in wavelength will be about 10 nm. Stated otherwise, if the absolute value of the resonant wavelength should have a precision of 1 nm or less, then the shift in the angle xcex8 of incidence should be 0.1 degrees or less.
The known optical receiver module, however, has the following drawbacks.
Firstly, the above-identified publication does not particularly point out a method of securing the optical fiber bundle 108 onto the semiconductor substrate 101 at a predetermined angle. Thus, it is difficult even for a skilled artisan to precisely define the angle xcex8 of incidence of the incoming light beam 109.
Secondly, according to the technique disclosed in the above-identified publication, the thickness of the spacer layer 103 is not controllable accurately enough. In general, a crystal-growing method for making the thickness variable relative to the surface of a substrate is already well known in the art (see U.S. Pat. No. 5,029,176, for example). However, this method is not precise enough to determine the absolute value of the resonant wavelength just as desired. In addition, it is usually hard to apply normal semiconductor device processing, in which a great number of devices are formed at a time on a single semiconductor wafer and then divided into respective chips after a wafer process, to the fabrication of the optical receiver modules. This is because a crystal-growing method allowing for a periodic thickness change of the wafer is needed in that case. But crystals can be grown in that manner just by a few methods among the numerous ones cited in the U.S. Patent identified above.
A first object of the present invention is allowing an incoming light beam to be incident onto the cavity filter of a demultiplexer or demultiplexer-receiver at a much more accurate angle.
A second object of the present invention is controlling the horizontal thickness change of at least one layer in the cavity filter precisely enough and thereby providing multiple selectable wavelengths for the filter through normal semiconductor device processing.
To achieve the first object, a first inventive demultiplexer includes: a semiconductor substrate; and a vertical cavity filter, which is formed on the principal surface of the substrate and transmits an incoming light beam with a predetermined wavelength. A resonant wavelength of the filter changes depending on at which point on the principal surface the light beam is incident. And the substrate has a recess with a slope that reflects or refracts the light beam and thereby makes the light beam incident onto the filter.
In the first demultiplexer, a slope that will make an incoming light beam incident onto a cavity filter is formed in a semiconductor substrate. The slope can be formed easily in the backside of the substrate, opposite to its principal surface, by a wet etching process, for example, so that an exactly predetermined angle is formed between the slope and the principal surface. In such a structure, if incoming signal light is incident onto the slope of the substrate, the light will be input to the vertical cavity filter while forming the predetermined angle with the principal surface and backside of the substrate with good reproducibility. As a result, a desired resonant wavelength is selectable very accurately.
In one embodiment of the present invention, the filter preferably includes: a first distributed Bragg reflector, which is formed out of a first semiconductor layer on the principal surface of the substrate; a spacer layer, which is formed out of a second semiconductor layer on the first reflector; and a second distributed Bragg reflector, which is formed out of a third semiconductor layer on the spacer layer. At least one of the first, second and third semiconductor layers preferably has its thickness changed relative to the principal surface of the substrate. In such an embodiment, a plurality of optical signals can be extracted from a multiplexed incoming light beam just as intended.
In another embodiment of the present invention, the slope is preferably a crystallographic plane that has been exposed by a wet etching process and has a predefined crystallographic plane orientation. In general, it is easy to expose a crystallographic plane of semiconductor crystals with a predefined plane orientation by wet etching. Accordingly, the slope of the substrate can also be easily defined to form a desired angle with the backside of the substrate, for example.
To achieve the second object, a second inventive demultiplexer includes: a semiconductor substrate; and a photonic crystalline layer, which is formed on the principal surface of the substrate and transmits an incoming light beam with a predetermined wavelength. A wavelength at an edge of a photonic band of the photonic crystalline layer changes in a direction parallel to the principal surface of the substrate.
The second demultiplexer includes a photonic crystalline layer instead of the vertical cavity filter of the first demultiplexer. As is well known in the art, photonic crystals form a band structure responsive to the energy of photons. Accordingly, photons, included in a special band gap called xe2x80x9cphotonic band gapxe2x80x9d, cannot exist in the crystals. That is to say, radiation with a wavelength corresponding to the energy contained in the photonic band gap is totally reflected by the photonic crystals. On the other hand, radiation with a wavelength corresponding to the energy not contained in the photonic band gap (i.e., the energy contained in the photonic bands) is transmitted through the photonic crystals. Accordingly, in the photonic crystalline layer, if the band edge of the photonic band gap changes parallelly to the surface of the substrate, then the range where the light beam is transmitted also changes. In this manner, multiple optical signals corresponding to desired wavelengths can be extracted sequentially.
In one embodiment of the present invention, the substrate preferably has a recess with a slope that reflects or refracts the light beam and thereby makes the light beam incident onto the photonic crystalline layer. In such an embodiment, a plurality of optical signals can be extracted just as intended from even a relatively densely wavelength-multiplexed incoming light beam.
In this particular embodiment, the photonic crystalline layer is preferably made up of a plurality of dielectric or semiconductor fine lines that are arranged like a lattice. And the width of each said fine line or a gap between adjacent ones of the fine lines preferably changes in a direction parallel to the principal surface of the substrate. In such an embodiment, the band edge of the photonic band gap of the photonic crystalline layer can be changed in the direction parallel to the principal surface. Unlike the vertical cavity filter, the thickness of the photonic crystalline layer is constant horizontally. Accordingly, the photonic crystalline layer can be formed by horizontal patterning and is far more compatible with normal semiconductor device processing in which a great number of devices with the same structure are formed at a time on a single wafer.
In another embodiment of the present invention, the slope is preferably a crystallographic plane that has been exposed by a wet etching process and has a predefined crystallographic plane orientation. In general, it is easy to expose a crystallographic plane of semiconductor crystals with a predefined plane orientation by wet etching. Accordingly, the slope of the substrate can also be easily defined to form a desired angle with the backside of the substrate, for example.
To achieve the first and second objects, a third inventive demultiplexer includes a horizontal cavity filter, which is formed on a substrate and transmits a part of an incoming light beam traveling in a direction substantially parallel to the surface. The part transmitted has a predetermined wavelength. The filter includes: a first distributed Bragg reflector, which is formed to make an optical path of the light beam parallel to the surface of the substrate; and a second distributed Bragg reflector, which is formed to be spaced apart from the first reflector.
The third inventive demultiplexer includes a horizontal cavity filter for selectively transmitting a light beam traveling in a direction substantially parallel to the surface of the substrate. Accordingly, when a light beam is externally entering the demultiplexer through an optical fiber bundle, the angle of incidence can be set to a desired one easily and just as intended. As a result, a ray with a desired wavelength can be easily selected from a densely wavelength-multiplexed incoming light beam. In addition, the horizontal cavity filter can be formed by horizontal patterning and is much more compatible with normal semiconductor device processing in which a great number of devices with the same structure are formed at a time on a single wafer.
In one embodiment of the present invention, each of the first and second distributed Bragg reflectors is preferably formed by arranging a plurality of dielectric or semiconductor thin plate members on the substrate at regular intervals so that the plate members cross the optical path. In this manner, the horizontal cavity filter is easily implementable.
In another embodiment of the present invention, a resonant wavelength of the filter preferably changes depending on at which point on the substrate the light beam is incident. In such an embodiment, a plurality of optical signals can be extracted from a multiplexed incoming light beam just as intended.
In this particular embodiment, the space between the first and second distributed Bragg reflectors preferably changes and is preferably tapered or stepped in the direction parallel to the surface of the substrate.
In still another embodiment, the third demultiplexer preferably further includes: a first photonic crystalline layer, which is formed between the substrate and the filter; and a second photonic crystalline layer, which is formed on the filter. In this case, if the photonic band gaps of the first and second photonic crystalline layers, sandwiching the horizontal cavity filter vertically, have such energies as covering the entire wavelength range of the incoming light beam, then the light beam entering the filter can be confined within the filter. As a result, the loss of the incoming light beam can be reduced.
To achieve the first object, a first inventive demultiplexer-receiver includes: a semiconductor substrate; a vertical cavity filter, which is formed on the principal surface of the substrate and transmits an incoming light beam with a predetermined wavelength; and a plurality of light-receiving areas defined on the filter. A resonant wavelength of the filter changes depending on at which point on the principal surface the light beam is incident. And the substrate has a recess with a slope that reflects or refracts the light beam and thereby makes the light beam incident onto the filter.
In the first demultiplexer-receiver, a slope is also formed in the backside of a semiconductor substrate as in the first inventive demultiplexer. In this structure, if a light beam is incident onto the slope of the substrate, the light beam will be input to a vertical cavity filter while forming a predetermined angle with the principal surface and backside of the substrate with good reproducibility. As a result, a desired resonant wavelength can be selected.
In one embodiment of the present invention, the first demultiplexer-receiver preferably further includes a light-absorbing layer and a window layer that are formed in this order on the filter. The window layer preferably includes a plurality of doped regions that are defined like islands. And the light-receiving areas are preferably respective parts of the light-absorbing layer that are located under the doped regions. In this manner, the respective optical signals, resulting from the demultiplexing by the vertical cavity filter, can be received just as intended. In addition, the areas at which the demultiplexed optical signals are received are integrated with the demultiplexer monolithically, and the demultiplexed optical signals (i.e., light beams) impinge through the slope onto the light-receiving areas. Accordingly, the window layer does not function as a window for transmitting the signal light therethrough but as a passivation film that can reduce leakage current.
To achieve the second object, a second inventive demultiplexer-receiver includes: a semiconductor substrate; a photonic crystalline layer, which is formed on the principal surface of the substrate and transmits an incoming light beam with a predetermined wavelength; and a plurality of light-receiving areas defined on the photonic crystalline layer. The substrate has a recess with a slope that reflects or refracts the light beam and thereby makes the light beam incident onto the photonic crystalline layer.
In the second demultiplexer-receiver, the second inventive demultiplexer is integrated monolithically with the light-receiving areas for receiving the optical signals resulting from the demultiplexing by the second demultiplexer.
Thus, the second demultiplexer-receiver can attain the same effects as those of the second demultiplexer.
In one embodiment of the present invention, the second demultiplexer-receiver preferably further includes a light-absorbing layer and a window layer that are formed in this order on the photonic crystalline layer. The window layer preferably includes a plurality of doped regions that are defined like islands. And the light-receiving areas are preferably respective parts of the light-absorbing layer that are located under the doped regions.